In a transmission, particularly an automatic transmission for a motor vehicle, the pressure is usually controlled as needed in the hydraulic circuit. Here the pressure level for the lubricating oil supply of the transmission parts can be kept low. Certain consumers, of course, need at times a pressure above a specific pressure value. During the shifting operations, for ex., the pressure is raised in the hydraulic circuit to make it possible to quickly fill the shifting elements with pressure medium.
For pressure control in hydraulic circuits of motor vehicles pressure control valves are customarily used to regulate control elements for the actuation of clutches. The control elements are controlled with the pressure-control valve by means of a proportional magnet consisting of at least one core, one coil and one armature. The coil current is controlled via the proportional magnet in proportion to the output quantity of power; the magnet armature and thus the element for clutch control are controlled according to the coil current. The pressure-current p/l characteristic curves needed for clutch adjustment are produced from characteristic magnetic power-current characteristic curves of the pressure control valve which is electronhydraulically controlled in the automatic transmission.
From the patent application DE 102 55 414 A1 is, at the same time, known a proportional pressure-control valve consisting of a magnetic part and a hydraulic part. The magnetic part consists at least of one coil, one armature and one core. The armature has here two parts, one first part being firmly connected with an armature bar and one second part being axially moveably arranged upon the armature bar. Both parts of the armature are interconnected by one spring. This design makes an increased gradient of the p/l curve possible. The gradient of the p/l curve is here very flat at low current values and the end of the curve is therefore high. Thereby the supply of pressure through the proportional pressure-control valve can be very precisely adjusted. Some consumers such as clutches in motor vehicles need as the control pressure, a pressure which is always above a first threshold value. Only when a pressure, abuts which is higher than the threshold value is applied, is it possible to control the consumer. The threshold value is called a charge pressure.
If a consumer has to be pressurized with charge pressure before it can be controlled, then a curve according to the prior art with the very flat area of the p/l curve would be needed so as to reach the charge pressure. To the flat gradient of the p/l curve a steep gradient of the curve follows. If now with this curve, the charge pressure of the shifting element must first be reached and only then can the controlling start, the area with steeper gradient, where the pressure can be very precisely controlled, is very much smaller. The shifting element therefore cannot be precisely controlled with such a curve.
The problem on which the invention is based is to make a proportional pressure-control valve which quickly enables the pressure to reach a first threshold valve and then increases the p/l curve.